The present invention relates to a scanning device and a scanning method for causing a beam of light to sweep across a surface and, more particularly, to such a device and method in which the beam is swept in succession along a plurality of generally parallel scan lines which are spaced apart on the surface.
Optical scanning devices for directing a beam of light to sweep across a surface are used in various applications. For example, color scanners are known which, by means of photoelectric sensors and color filters, analyze a color photograph on a point by point basis to produce a color component electrical signal for each of a series of color separations. The signal is used to control the exposure of a photographic film which forms the color separation. The film is exposed, point by point, to light which is modulated in accordance with the color component electrical signal such that the color separation reflects the color component density at the scanned points on the original photograph.
Similar scanning is utilized to reproduce original images or paste-up images on printing or photographic plates for single color reproduction. Additionally, optical scanning by sweeping a beam of light across a surface is utilized in facsimile and character recognition systems.
In known scanners, a beam of light, such as for example a beam of laser light, is produced by a stationary light source. A moving beam deflector device, positioned in the path of the laser beam, diverts the beam to the surface which is to be scanned. The moving beam deflector device may for example comprise a mirror or prism arrangement which is rotated, thus causing the beam to sweep across the surface along a scan line or path. Relative movement between the beam diverting device and the surface to be scanned is accomplished such that successive sweeps of the beam are made along a series of spaced scan lines on the surface.
Imperfections in the beam diverting device and its bearing supports may, however, introduce errors in the path of the deflected beam, thus resulting in improperly positioned scan lines. A number of different approaches to eliminating or reducing such errors have been taken in prior art scanning devices. One such approach is incorporated into a scanning device disclosed in U.S. Pat. No. 4,002,830, issued Jan. 11, 1977 to Brown et al, which scans by means of a rotating polygonal mirror. A beam is directed to the rotating polygonal mirror by a reflecting element which is adjusted in position by a servo control system. The position of the reflecting element is altered to correct for variations in the angles between the facets of the rotating polygonal mirror. The Brown et al scanning device cannot however, correct for errors induced by other factors, such as bearing wobble.
A somewhat similar correction arrangement is shown in U.S. Pat. No. 4,268,110, issued May 19, 1981, to Ford. In the Ford scanner, a multi-faceted rotating optical scanning element deflects light from a laser source to a surface to be scanned. Associated with each facet of the element is a transparent correction element which defines a slight wedge angle between its two surfaces. By adjusting the orientation of each of the correction elements, errors in the angular orientations of the facets may be compensated. The Ford correction arrangement, however, is not capable of compensating for errors induced by bearing wobble.
Other prior art scanners correct for scanning element wobble by utilizing a rotating pentaprism as the scanner element. Light directed to the pentaprism is reflected at a constant angle, typically 90.degree., regardless of small changes in the orientation of the pentaprism. Rotating the pentaprism about an axis aligned with the incident beam of light produces a rotating beam which can scan across a surface. If the pentaprism wobbles during this rotation there is no effect of such wobble on the angle of exit of a scanned beam from the pentaprism because of the reflection pattern of the light within the prism. U.S. Pat. No. 3,875,587, issued Apr. 1, 1975, to Pugsley, discloses such a scanning system.
While the use of a rotating pentaprism does compensate for wobble, nevertheless the scanning operation which can be accomplished using a single pentaprism is somewhat limited. If a pentaprism is continuously rotated at a uniform rate, it can either be used to scan a surface only during a limited portion of each rotation, thereby reducing appreciably the number of lines which can be scanned per unit time, or the surface to be scanned must be wrapped around the pentaprism, thereby limiting the type of surface which can be scanned, and its dimensions. If the pentaprism is not rotated continuously, but rather is cyclically rotated only through a limited angle and then returned to its original position, problems may be encountered in accomplishing this quickly and accurately due to the mass of the pentaprism.
It is seen, therefore, that there is a need for a simple, accurate optical scanning system and method for deflecting a beam of light to scan across a surface in a rapid, efficient manner.